Two known types of compressors are the wobble piston type and the swashplate type. The wobble piston type is exemplified by U.S. Pat. No. 3,961,868 issued Jun. 8, 1976, to Droege, Sr., et al. for xe2x80x9cAir Compressorxe2x80x9d. Such a compressor uses a piston whose head has a peripheral seal that seals with a cylinder bore. The piston rod is mounted radially on a crankshaft. The piston includes no joints or swivels. As a result, the piston head is forced to xe2x80x9cwobblexe2x80x9d in two dimensions within the cylinder bore as it is driven by the crankshaft.
The swashplate type compressor uses a plurality of axial cylinders arranged in a circle about a drive shaft. A swashplate is inclined relative to the shaft axis such that the plate gyrates as the drive shaft is rotated. Pistons are mounted in each of the cylinders. The ends of the piston rods are connected to elements that slide over the surface of the swashplate as the swashplate rotates. The result is that the centerline of the piston head is moved solely in an axial direction as the pistons are stroked within the cylinders. An example of such an axial piston swashplate compressor is found in U.S. Pat. No. 5,362,208 issued Nov. 8, 1994 to Inagaki, et al. for xe2x80x9cSwashplate Type Compressorxe2x80x9d. Another example is U.S. Pat. No. 4,776,257 issued Oct. 11, 1988, to Hansen for xe2x80x9cAxial Pump Enginexe2x80x9d. In the Hansen patent, the centerline of the piston heads are inclined relative to the centerline of the cylinder bore, but the piston heads are moved only along the piston head centerline in one direction.
The present invention combines the wobble pistons normally used in radial piston pumps with a nutating plate rather than the swashplate normally used in axial piston pumps. The result is a simple and effective fluid pumping apparatus.
In accordance with the invention, a fluid pumping apparatus includes a drive shaft and a cylinder having a bore. Fluid inlet and outlet valves communicate with the cylinder bore. A bearing is mounted on the shaft with the centerline of the bearing at an angle to the shaft axis. A piston carrier is mounted on the bearing. A wobble piston is rigidly attached to the arm and is disposed in the cylinder bore. As the drive shaft rotates, the centerline of the bearing will precess about the shaft axis, and the arm will be moved, thereby causing the wobble piston to move in three dimensions within the cylinder bore.
In one aspect of the invention, the piston is supported by a leaf spring, which helps control the movement of the piston and reduce the bearing loads. Multiple pistons and leaf springs are preferably provided, and the leaf springs are prevented from rotating in a plane perpendicular to the shaft axis.
In another aspect, the pistons are connected to the piston carrier by radially resilient but axially stiff connecting rods. The axial stiffness of the connecting rods is sufficient to exert the required forces of compression and vacuum on the piston without significant change in length of the rod, but is radially resilient so as to reduce the radial loads exerted on the piston seal, and therefore increase the life of the piston seal.
In another aspect, particularly where multiple pistons are employed operating in phased relationship with one another, the piston carrier, leaf springs and open ends of the cylinders are enclosed to reduce noise. A filter opening may be provided in the enclosure, which is necessary if intake is through the pistons as is preferred. The enclosure preferably does not enclose the outside surfaces of the cylinders, so as to permit cooling air to circulate around them.
In another preferred aspect, one end of the cylinders are seated against a housing and the housing supports bearings which support the shaft so as to cantilever the rotor of the motor inside the stator, with the stator mounted to the side of the housing opposite from the crankcase of the pump. A cylinder retainer is seated against the opposite ends of the cylinders and is fixed to the housing to clamp the cylinder, to the housing. The cylinder retainer preferably includes a tapered lead-in surface into the open ends of the cylinders. The cylinder retainer also defines cavities around the leaf springs, and a cover mates with the cylinder retainer to enclose the crankcase to reduce noise. Multiple cylinders are arranged in phased relationship with one another so that the volume of the crankcase stays substantially constant as the pistons reciprocate in the cylinders.
In another aspect of the invention, the inlet valves are provided in the pistons and the outlet valves are provided in the housing. A head over the outlet valves defines an exhaust chamber common to all of the cylinders and provides an outlet port.
In another preferred aspect, the xe2x80x9ctopxe2x80x9d surface of each cylinder is in the shape of a section of a cone, so as to minimize the clearance volume of the cylinder as the piston moves through its top dead center position.
It is a principal object of the invention to provide a simplified axial piston pumping apparatus using wobble pistons.
It is another object of the invention to provide an axial piston pump of quiet operation, efficient power usage and good longevity which does not require the use of sliding elements requiring continuous lubrication.
The foregoing and other objects and advantages of the invention will be apparent from the following detailed description. In the description, reference is made to the drawings which illustrate preferred embodiments of the invention.